High concentrations of hydrogen dissolved in construction and fabrication materials are an ongoing concern because hydrogen concentrations can alter the mechanical properties of the materials causing cracking, embrittlement, weakening, and other detrimental changes. Hydrogen may be introduced into materials by a number of processes including, for example, arc welding, extended exposure to pressurized gases containing hydrogen, various corrosion processes, and repeated exposure to high pressure gases containing hydrogen, such as in cannon barrels. Hydrogen damage is of particular concern with the use of metals, and, in particular, steels which are susceptible to hydrogen embrittlement, hydrogen induced cracking (HIC), hydrogen assisted cracking (HAC), and other hydrogen-induced damage. Generally, there is an acceptable concentration of introduced or dissolved hydrogen for each material above which the material is considered unsafe or unsatisfactorily weak for its intended use.
The buildup of hydrogen in steel is of particular concern in fabrication and construction processes relying heavily on arc welding. During arc welding, atomic hydrogen is produced in the arc by the decomposition of hydrogenous compounds, such as water, lubricants, or molecular hydrogen in the air or base metal, which enter the arc. The atomic hydrogen is soluble in the liquid weld pool or bead and is retained within the weld material as it freezes or solidifies. However, a portion of the hydrogen, i.e., diffusible fraction, rapidly diffuses out of the metal even at normal room temperatures. The diffusible fraction is generally accepted as a primary indicator of potential hydrogen damage of the welded joint, with initial hydrogen concentrations being particularly useful in predicting damage. Therefore, measurement of diffusible hydrogen concentrations can provide an effective determination of whether a welded joint has a hydrogen concentration that is below an acceptable concentration limit, e.g., determining, at least in part, the quality or strength of the welded joint.
Current industry practice for assuring the quality of welds involves the development of a standard welding procedure, which is then followed during all welding processes. Under ANSI/AWS A4.3-93 “Standard Methods for Determination of the Diffusible Hydrogen Content of Martensitic, Bainitic, and Ferritic Steel Weld Metal Produced by Arc Welding” by the American Welding Society, a welding procedure is qualified by welding four (4) samples or coupons of a particular steel, promptly quenching the steel coupons to low temperatures, and then testing the steel coupons for diffusible hydrogen concentration. The diffusible hydrogen concentration is measured in units of volume of gas per weight of deposited metal, e.g., milliliters of hydrogen per 100 grams of deposited weld metal. The volume of hydrogen is measured under current standard procedures either by volumetric displacement of mercury by placing the sample in a audiometer and allowing hydrogen to diffuse from the sample for at least 72 hours or by baking each sample in a sealed container for an extended period to evolve gases and then analyzing the gases in a gas chromatograph to identify the volume of hydrogen.
While such methods of measuring the volume of diffusible hydrogen in samples provide relatively accurate measurements of the diffusible hydrogen content in each sample, they do have a number of significant limitations and problems. First, these methods measure bulk or total diffusible hydrogen evolving from the sample and do not provide a method of identifying concentrations of hydrogen or, more particularly, localized concentrations of hydrogen. Such, localized concentrations of hydrogen, when combined with residual stresses at inclusions, grain boundaries, or the weld fusion lines, can cause cracks to occur even though overall diffusible hydrogen for a bulk sample may be below allowable content limits. Localized concentrations of hydrogen often occur more readily in higher strength steels, which have a lower allowable hydrogen content limit, e.g., as low as 1 to 2 ml/100 g. Second, these current standard methods do not lend themselves to nondestructive field or in-place testing of welded joints in components and structures. Therefore, only samples or blanks of welded metal, not actual welds intended for use in machinery, pipelines, and the like, can be measured. Third, once a standard welding procedure is approved with these measuring methods, the welding procedure must be taught and closely followed by every welder. If a welder does not precisely follow the welding procedure due to poor training or other causes, the resulting welded joints in actual machinery, pipelines, or other unacceptable welded objects would not be detectable with these methods, which may leave welded joints intended for use that have undetected hydrogen concentrations above the allowable limits and may fail catastrophically. Fourth, these methods are expensive and time consuming. Delays of at least 24 hours for chromatography testing and delays of at least 72 hours for mercury displacement testing are common.
Some efforts have been made to develop sensors for other applications that detect the presence of hydrogen but are not useful in measuring the concentration of hydrogen. These sensors utilize chemochromic reaction, i.e., a reaction causing optical properties to be altered when certain transition metal oxides are exposed to hydrogen. For example, U.S. Pat. No. 5,708,735 issued to Benson et al. discloses a hydrogen leak detector for hydrogen fuel tanks, which can be placed near the hydrogen fuel tank to monitor the space near the hydrogen fuel tank for the presence of hydrogen. The patented Benson et al. hydrogen leak detector transmits an alarm signal when hydrogen is detected in the space. The detector includes a sensor having an optical fiber with a beveled, three-faceted end that is coated first with a conductive metal (gold or silver) and then a transition metal oxide, such as tungsten oxide. A catalyst material is applied to the metal oxide to quicken the reaction with hydrogen, and finally, a polymer layer is applied over the catalyst to provide a barrier against contaminants. The order and the materials in these layers and the use of a beveled end were selected to produce a detector that utilizes guided wave resonance phenomenon to increase the sensitivity and quickness of hydrogen detection. Resonance occurs when light from an included white light source passes along the optical fiber and strikes the metal-coated, faceted end at an angle just above the critical angle for total internal reflection. The evanescent wave stimulates resonant absorption of the light by free electrons in the metal to produce a surface-plasmon. The layer of transition metal oxide, i.e., chemochromic material, provides an optical wave-guide for light at the surface-plasmon resonance. The layered coating produces a coupled resonance at the surface-plasmon wavelength that is very sensitive to the optical constants of the transition metal oxide layer. When hydrogen reacts with the metal oxide, the resonance frequency shifts, and this shift is detected by a monitoring device that analyzes the spectrum of the reflected beam and an alarm signal is transmitted. While providing initial detection of the presence of a small amount of hydrogen in the general volume surrounding the sensor, this leak detector is not useful for accurately measuring diffusible hydrogen concentrations in a material sample for identifying potential hydrogen damage.
Consequently, a nondestructive, yet accurate, method and device for measuring concentrations of diffusible hydrogen quickly at various locations on material samples, e.g., along the length of a welded joint on a machine, pipeline, or other article that is intended to be placed into use, would be useful to and well-received by the construction, fabrication, and related industries. Further, such a method and device would preferably provide other advantages over the current equipment intensive laboratory tests, such as being portable, inexpensive, and easy to use, to assist industries in more readily meeting quality assurance and safety requirements and standards.